In closed containers, a liquid often changes its volume, for example, as a result of temperature fluctuations. As a consequence, a positive or negative pressure is produced in the container, which may produce severe damage or danger, depending on the function of the container. Free air/gas volume in the container may also fluctuate. Additional problems are created if the liquids themselves de-gas.
Measuring devices, for example, are filled only with 90 to 95% of liquid, such as glycerole silicone oil. As temperature changes the volume of the liquid, the system is damped as the liquid expands into the remaining space. The expansion of the liquid causes gas to exert pressure on the container wall or devices within the container. As a result, delicate measuring devices may provide false or inaccurate measurement results.
To solve this problem, valve systems have been used with limited success in some applications to regulate pressure exchanges. If the valves are continuously open, however, liquid may run out of the housing in certain positions. Such an effect is not desirable in measuring device applications. Positive pressure valves have been used to prevent liquid spillage, but they do not allow for gas admission, which may be required, for example, after a container or housing has cooled down. Bidirectionally acting valves are feasible, but very costly; therefore less effective valves are chosen.
To accomodate volume changes, one or more of the walls of a container or housing may be elastically deformable to adapt to positive and negative pressures within the housing, however, such elastically deformable walls considerably restrict the range of applications of such a container or housing. Accordingly, elastically deformable walls are typically only used in applications where only slight pressure differences are to be expected.
It is known for a closed housing containing a liquid to be provided with at least with one opening, which is covered by a cover consisting of a liquidproof, yet gas permeable material to obtain a pressure equilibrium. A cover of this type may consist of a porous tetrafluoroethylene polymer which works like a valve performing a bidirectional gas exchange. In this way, the housing can be ventilated or deventilated, depending on what is required, without the danger of liquid spillage.
Furthermore, it is known to provide a lid in the form of a screw-on lid having a pressure compensation device with a porous, gas-permeable yet liquidproof membrane and having a surge protection element, which consists of a gas permeable material and is designed such that it breaks the surge pressure exerted by the membrane on the pressure compensation device.
The foregoing are manufactured as separate components which are inserted or installed in the lid after the lid has been produced. This makes production relatively complex so that lids of this type are rather expensive to manufacture.
The foregoing illustrates limitations known to exist in present lid constructions. Thus, it is apparent that it would be advantageous to provide an improved lid directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.